Deep astronomical images, like those used in extragalactic studies, seriously suffer from noise in the data. Generally speaking, the sources of noise in an astronomical image are photon counting noise and Instrumental noise which are discussed in Photon counting noise and Instrumental noise. This review finishes with Generating random numbers which is a short introduction on how random numbers are generated. We will see that while software random number generators are not perfect, they allow us to obtain a reproducible series of random numbers through setting the random number generator function and seed value. Therefore in this section, we will also discuss how you can set these two parameters in Gnuastro’s programs (including the arithmetic operators in Random number generators).